An illumination optical system for microlithography is known from US 2006/0132747 A1. The concept of a specular reflector is described therein, in which the position of a facet mirror used to preset illumination is not (in contrast to some other known illumination optical systems) limited to an entry pupil plane a following projection optical system or a plane which is optically conjugated with the entry pupil plane of the following projection optical system.